1. Field of the Invention
This invention relates to valves designed to control the flow of water through hydraulic turbines. More particularly, this invention belongs to the category of valve controls known as wicket gates, which are used to direct high speed water flow against turbine vanes.
2. Description of Prior Art
Hydro-power generating systems are dependent on a continuous source of water. This source of water is commonly drawn from an open reservoir, such as a river, a lake, or an ocean. In general, the water intake requirements can amount to many thousands of gallons per minute. Turbines of the type used to transform the hydrostatic pressure associated with such high water flow rates into electric power are equipped with adjustable valves referred to as wicket gates. Pairs of these wicket gates are placed in the penstock tube--the conduit that is used to carry water from its source to the turbine. Typically there are ten pairs of wicket gates to a turbine. The gates direct water to the vanes of the turbine, whereby the water motion is converted to rotary turbine motion. The wicket gates are used to provide regulation of the power output of the turbine by directing water into the turbine at whatever flow rate is demanded at a particular time. This regulation is achieved by adjusting the openings of the wicket gates, thereby providing a particularly useful means of compensation as the hydrostatic head varies with the filling or emptying of a storage reservoir. This regulation is also useful in modulating sudden flow surges to the turbine when it is started and stopped.
Additionally, in the distinct class of water-moving devices known as reversible pump-turbines--wherein water motion may be converted to mechanical motion and, in reverse, mechanical motion may be converted to water motion--the wicket gates are capable of regulating the flow of water from the turbine runner back through the penstock tube. This is particularly useful when a sudden loss of power to the generator occurs and back-flow must be prevented.
Throughout the course of operation of the turbine, the demand for water may vary between the maximum intake possible, to almost no water at all--within a short period of time. At high water flow rates the wicket gates are plagued by problems of erosion and cavitation pitting. At low or no water flow, marine growth and corrosion plague the submerged gates. These problems directly alter the surface profile of the gates, resulting in a reduction in the effective water flow rate through the penstock tube to the turbine vanes. Wicket gates are designed with shapes that permit optimized water flow, to the vanes, with minimal turbulence. Corruption of the gate surface by erosion and cavitational pitting, as well as by marine growth, increases turbulence and thereby decreases the water flow rate.
A further problem, which occurs in cold water regions only, is the formation of ice on underwater structures such as wicket gates. Ice formation may occur at high as well as at low--or no--water flow rates. In general, ice borne along by flowing water, or ice formed at nucleation sites on the structure, may build up and reduce the effective flow rate of water past the gate. For a more thorough discussion of the problems associated with ice build-up on underwater hydroelectric plant structures attention is called to U.S. Pat. No. 4,846,966.
Current wicket gates are fabricated of cast iron or steel, metals that give rise to the problems noted above. In particular, if left untreated, they quickly corrode in salt water. During periods of low water flow they provide an ideal surface for marine growth--which must be removed to maintain the desired surface profile. Current environmental regulations limit the use of paints with the most effective anti-fouling properties, and the use of less-effective, but environmentally safer paints has shortened the required maintenance interval for the hard-to-reach wicket gates.
In addition to the low flow rate problems, there are the problems associated with high water flow rates--primarily erosion and pitting caused by cavitation. Erosion in this context, involves the gradual wearing-away of the surface of a submerged structure. It is caused by the abrasive effect of impinging particles that are carried along by rapidly moving water. Pitting, on the other hand, occurs when gas-filled cavities, created by structure vibration, collapse on the surface of the structure. This collapse imparts a pressure pulse to the surface which is of such magnitude that it actually removes small pieces of metal. A pitted surface profile results, causing more turbulence and associated vibration, thereby accelerating the cavitation-caused damage.
In order to operate the turbine at its optimal level, it is therefore necessary to perform regular maintenance procedures on the gates. However, due to their location well within the hydro-power system, it is extremely difficult to gain access to them. Any maintenance requires shutting down the power-generation operation and sending divers into the penstock tube. The divers are required to inspect, and, if possible, to make repairs in situ. When such repairs are not possible, the gates must be removed and brought through the penstock tube and up to the water surface. Due to the dimensions and weight of the gates, they must be removed individually. Because of the awkwardness of the retrieval operation, the gates are dragged through the tube, often resulting in damage to the gates, the tube, or both.
To circumvent the effects of marine growth, ice build-up, corrosion, erosion and cavitation, metals resistant to these effects have been utilized. For example, utilizing bronze will slow the deleterious effects considerably, as compared to the use of cast iron. The problem is that known erosion- and cavitation-resistant metals are very expensive and are often extremely difficult to machine to the close tolerances required to achieve optimum water flow-through. Any overall comparison between an expensive yet resistant material and a less-resistant material generally results in selection of the latter even though such selection means more frequent repairs and replacements.
Another approach to minimizing the effects of cavitation is through the shaping of the turbine components, including the wicket gates, so as to increase the maximum flow rates achievable with minimum turbulence (i.e., minimum turbulence at the boundary between the submerged body and the flowing water). Although these efforts ameliorate the effects of cavitation--by reducing the formation of bubbles at the boundary layer--they do not eliminate them; thus periodic maintenance and replacement of the wicket gates is still required. Other design optimization techniques include selection of the location of the turbine and control of the turbine operation. Changes in these parameters may result in a reduction in the effects of erosion and cavitation, but such a reduction often comes at the expense of the power-generating capacity of the hydroelectric unit. The net effect of such an effort is an extension of the maintenance life-cycle and a reduction in power output.
In recent years viscoelastic materials, with their favorable strength-to-weight ratio and corrosion-resistance capacity, have been used to extend maintenance life cycles without encroaching upon the turbine operating parameters. These materials are considered to be effective in reducing cavitation because of their capacity to attenuate vibrations. However, viscoelastic materials have not been considered for use in the fabrication of the water-directing components of hydro-power generating systems, principally due to the concern that they would be unable to resist the tremendous forces exerted by rapidly flowing water. However, such materials have been considered for use in applications similar to, but much smaller than the wicket gates of hydroelectric turbines including, for example, small fixed-blade pumps.
In particular, Lobanoff (U.S. Pat. No. 3,876,327) discusses the advantages of fabricating pumps of viscoelastic materials, including their impact- and corrosion resistance, and also discusses the disadvantages of pumps comprising plastic-coated metal parts. Also, Wilkinson (U.S. Pat. No. 3,588,267) discloses stationary air-foil blades, made of a high-strength viscoelastic material, that are positioned between two aluminum hubs, and used in the design of a jet engine.
To overcome the deficiencies associated with the present wicket gates--so as to reduce the frequency of maintenance procedures--what is needed is a wicket gate that is: (1) formed of a material capable of resisting the surface profile distortions caused by marine growth, ice build-up, corrosion, erosion, and pitting; (2) durable enough to withstand, without distortion, the stress imposed by the high-pressure flow of thousands of gallons of water per minute; (3) sufficiently "damped" (i.e., energy absorbent) to minimize structural vibrations that cause cavitation; and (4) lightweight enough to minimize hydraulic system fatigue.